WASHINGTON, Jan 17 (Reuters) - Michelle Obama on Friday celebrated a rite of passage for Americans turning 50: getting a membership card to AARP, a national organization that promotes the interests of older people.

The first lady tweeted a photograph of herself, smiling while holding up a card from the group, which with its approximately 38 million members cuts an influential swath in the nation's capital.

"Excited to join Barack in the 50+ club today ... check out my @AARP card," she said in a post signed with her initials.

Her husband, President Barack Obama, is 52.

AARP, previously the American Association of Retired Persons, now goes simply by its acronym. One of the most powerful lobbying groups in the country, it was a strong backer of the president's signature health care law, the Affordable Care Act.

People are eligible for AARP membership on their 50th birthday, even though most Americans do not retire until their mid-60s. But you do not need to be a retiree to join, and getting the group's membership offer, for some, triggers dark humor about the inexorable march of time and the ravages of age.

The first lady is an example of how youthful people still are at 50, said AARP chief executive Barry Rand.

"Not only is Mrs. Obama helping to change and reshape the way people look at aging in this country, but she's also showing folks how great 50 looks today," he said in a statement.

"We're proud to be able to call her a member of the AARP family and know that she will continue to set a great example to all by showing that age sets no boundaries on what you can achieve in life," Rand said.

The group has not always been on the same page as the administration. It previously described as "irresponsible" one of the president's budget proposals to rein in the costs of retiree programs by linking benefit increases to a less generous measure of inflation.

Michelle Obama's 50th birthday has drawn additional public scrutiny to the first lady, previously a lawyer and a hospital executive, who has used her White House position to launch a campaign called "Let's Move" to fight childhood obesity.

In an interview with People magazine, she said she aspires to remain fit and active into her 70s and 80s.

Asked what she thought about Botox or plastic surgery, she said she could not envision such procedures for herself, but that she has learned to "never say never."

And she still likes to dance. Her invitation to a birthday dance party advised guests to "wear comfortable shoes," the magazine reported. (Reporting by Mark Felsenthal, editing by G Crosse and Ken Wills)